Domain inversion, which is a phenomenon whereby the polarization of a ferroelectric is forcibly inverted, can be utilized to form periodic domain inversion regions (domain inversion structures) in the interior of a ferroelectric. The domain inversion regions thus formed are used in optical frequency modulators that make use of surface elastic waves, optical wavelength conversion elements that make use of the inversion of nonlinear polarization, optical polarizers that make use of a prism- or lens-shaped inversion structure, and the like. In particular, if the nonlinear polarization of a nonlinear optical substance could be periodically inverted, it would be possible to produce optical wavelength conversion elements of extremely high conversion efficiency. If these could be used to convert the light from semiconductor lasers and so forth, it would be possible to obtain a compact short-wavelength light source that could be applied to printing, optical information processing, applied lasermetrics, and other such fields.
Methods that have been reported in the past for forming periodic domain inversion regions include a method involving the thermal diffusion of titanium, a method in which SiO2 is loaded and then heat treated, and a method in which proton exchange and heat treatment are performed. Meanwhile, another has been reported for forming periodic domain inversion regions by taking advantage of the face that the spontaneous polarization of a ferroelectric is inverted by an electric field. Methods involving this electric field include, for example, a method in which the −C plane of a substrate cut along the C axis is irradiated with an electron beam, and a method in which the +C plane is irradiated with positive ions. In either case, domain inversion regions with a depth of a few hundred microns are formed by the electric field formed by irradiation with charged particles.
Another conventional method for manufacturing domain inversion regions that has been reported is to form an interdigital electrode on an LiNbO3 substrate and applying a pulsed electric field to this (see Japanese Laid-Open Patent Application H3-121428 and Japanese Laid-Open Patent Application H4-19719, for example). With these methods, a periodic interdigital electrode is formed on the +C plane of the LiNbO3 substrate, and a flat electrode is formed on the −C plane. The +C plane is then grounded, and pulse voltage, typically with a pulse width of 100 μs, is applied to the −C plane by a pulsed light source and thereby apply a pulsed electric field to the substrate. The electric field that is required to invert polarization is approximately 20 kV/mm or higher. In the application of an electric field of this value, there is the possibility that the substrate may be damaged by the field if the substrate is too thick. If the thickness of the substrate is kept to about 200 μm, however, crystal damage by field application can be avoided, and it will be possible to form domain inversion regions at room temperature. A deep domain inversion structure that goes all the way through the substrate is obtained.
It has also been disclosed that a periodic domain inversion structure can be formed by forming an interdigital electrode on a Z-cut magnesium-doped LiNbO3 substrate (hereinafter referred to as MgLN), and applying voltage to this (see Japanese Laid-Open Patent Application 2001-66652, for example).
To increase the efficiency of an optical wavelength conversion element, a short-period domain inversion structure with a period of 3 to 4 μm is necessary. When domain inversion regions are formed by the application of an electric field, after the polarization is inverted directly under the electrode, the domain inversion regions spread out parallel to the surface of the substrate. This makes it difficult to shorten the period of the domain inversion structures. To solve this problem, with a conventional method, a brief pulse of voltage, with a pulse width of about 100 μs, is applied, which shortens the voltage application time and forms a short-period domain inversion structure.
Another known method for forming a short-period domain inversion structure is to form a groove on the surface of a Z-cut LiTaO3 substrate to suppress the expansion of the domain inversion in the lateral direction, and thereby form domain inversion regions with a period of 3.8 μm (see Japanese Laid-Open Patent Application 2000-147584, for example).
Yet another method for forming a short-period domain inversion structure is to form domain inversion regions separately by dividing up into a plurality of domains in order to form domain inversion microstructures within a dielectric material, and thereby form short-period domain inversion at a period of 4 μm, for example (see Japanese Laid-Open Patent Application 2003-307758, for example).
A method in which a short-period domain inversion structure is formed in MgLN has also been proposed (see Japanese Laid-Open Patent Application H6-242478, for example). With this method, a periodic domain inversion structure is formed in Z-cut MgLN. More specifically, with this method, an interdigital electrode is formed in the +Z plane of MgLN, and corona irradiation is performed from the back to form a periodic domain inversion structure. The result is a domain inversion structure in which the period is 4 μm and the structure goes all the way through the substrate thickness of 0.5 mm.
Another known method is to sandwich an SiO2 film between the −Z plane of Z-cut MgLN and an electrode, which prevents damage to the substrate and forms a domain inversion structure that goes all the way through the substrate thickness of 0.3 mm at a period of 5 μm (see Japanese Laid-Open Patent Application H7-281224, for example).
A method in which a domain inversion structure is formed in off-cut MgLN has also been proposed (see Japanese Laid-Open Patent Application H9-218431, for example). With this method, an electrode is formed on an off-cut substrate, and voltage is applied to this, which allows a needle-like domain inversion structure to be formed. The domain inversion structure grows in the direction of polarization of the crystals. A domain inversion structure with a period of about 5 μm can be formed in an off-cut MgLN substrate. The domain inversion is formed in a needle shape in the interior of the substrate, using an off-cut substrate in which the polarization direction has been slightly tilted from the substrate surface.